First of all, it absolutely kneads the dough. I just made two loaves last night that were better kneaded than anything I've made by hand. America's Test Kitchen has done tests with various mixers and have had success kneading doughs. Perhaps we have different definitions of "kneading", but as far as developing gluten in bread dough, my mixer does just fine. There are hundreds of videos of people successfully using theirs to develop gluten in bread dough as well.
I can't speak to all of KitchenAid's mixers, but the new 6qt bowl-lift Pro 600 mixer has sintered metal gears, not nylon. The only warning about the dough hook in the manual is to only use the dough hook on speed 2.
"Note: Use Speed 2 to mix or knead yeast doughs. Use of any other speed creates high potential for Stand Mixer failure. The PowerKnead (tm) spiral dough hook efficiently kneads most yeast dough within 4 minutes."
The only mention of extended mixing times is in the troubleshooting guide: "Under heavy loads with extended mixing time periods, you may not be able to to comfortably touch the top of the Stand Mixer. This is normal."
I didn't time how long I ran my mixer for my bread last night, but I did check to see if the housing was getting hot, and it was just mildly warm near the bottom of the housing (though for a 6 quart mixer and only 5 cups of flour, I wouldn't really expect it to be having much trouble. It was essentially only working a 50% duty cycle because the dough tends to only contact the sides a couple of times per rotation)
If the gears DO strip outside of warranty, they're like $25 on Amazon and take all of 15 minutes to replace. I did a lot of research before buying this mixer - $500 for a kitchen appliance is a big deal - and the quality of materials and repairability were the key deciding factors for this model.
I'm sure what you're saying is true for some Kitchenaid mixers (there are replacement nylon gears available for some kitchenaids, for example), but it's certainly not true for all.